1. Technical Field
This disclosure generally relates to illumination, and more particularly to apparatus and methods that employ a heat exchanger to cool solid-state light sources.
2. Description of the Related Art
With increasing trend of energy conservation and for various other reasons, solid-state lighting has become more and more popular as the source of illumination in a wide range of applications. As generally known, solid-state lighting refers to a type of lighting that emits light from a solid-state materials, such as a block of semiconductor material. Such contrasts with more traditional forms of lighting, for example incandescent or fluorescent lighting which typically employ a filament in a vacuum tube or an electric discharge in a gas filled tube, respectively. Examples of solid-state light sources include light-emitting diodes (LEDs), organic light-emitting diodes (OLEDs), and polymer light-emitting diodes (PLEDs). Solid-state lighting devices typically require several solid-state light sources to produce a suitable level of illumination.
Solid-state light sources tend to have increased lifespan compared to traditional lighting. This is because solid-state light sources have a greater resistance to shock, vibration, and wear. Solid-state light sources generate visible light with reduced parasitic energy dissipation (i.e., reduced heat generation) as compared to traditional lighting. Nevertheless, solid-state light sources do generate heat and excess heat needs to be removed to protect the solid-state lighting sources from damage.
Some illumination devices use a heat exchanger (e.g., heat sink) to remove heat from the light sources. Heat exchangers such as heat sinks are typically made of materials with high thermal conductivity, for example metals such as aluminum and copper, and which are also electrically conductive. Less commonly, heats exchangers may be electrically non-conductive, to provide electrical isolation with respect to the various components of the illumination device. Electrically non-conductive heat exchangers are typically made of an electrically non-conductive polymer loaded with electrically non-conductive particles such as boron nitride or other ceramic materials. However, electrically non-conductive heat exchangers tend to have very low thermal conductivity relative to electrically conductive heat exchangers. Electrically non-conductive heat exchangers are also typically more expensive than electrically conductive heat exchangers.
Solid-state light sources are typically soldered or adhesively bonded to a printed circuit board (PCB). The PCB may, for example, take the form of a “metal core” type or an “enhanced FR-4” type PCB. The PCB is then attached to a heat exchanger via with multiple screws or via adhesive. When screws are used, a thermal interface material (e.g., silicone grease, thermal pad, graphite sheet) is typically placed between the PCB and the heat exchanger. The thermal interface material must be compressed with sufficient force to adequately conduct thermal energy away from the solid-state light sources. The screws must be positioned close to the solid-state light sources to assert a high force in order to achieve the high degree of intimate contact required to provide a suitable level of thermal conduction. The large number of solid-state light sources required to achieve a desired level of illumination means that a large number of screws are typically required to fasten the PCB tightly enough to the heat exchanger to obtain the desired level of thermal conduction. For example, one commercially available lamp having 12 Cree XPG LEDs mounted on a metal core PCB employs 35 screw fasteners to couple the metal core PCB to the heat exchanger.
As noted, the PCB may be physically coupled to the heat exchanger via an adhesive. Such an approach suffers from a number of drawbacks: For example, typical adhesives do not provide as high a thermal conductivity as compared to liquid silicone thermal interface materials or thermal interface pads made of silicone or graphite. Also, adhesive attachment of the PCB to the heat exchanger is permanent. Consequently, it is imperative that the PCB is correctly positioned the first time it is applied to the heat exchanger. Also, adhesives are typically not electrically insulating, unlike silicone thermal pads. Thus, the heat exchanger is not electrically isolated from the PCB. Electrical isolation is desirable in many designs to protect a user from electrical shock. Additionally, the PCB is not easily removable for servicing or replacement if one or more LEDs is damaged, out of specification or fails.
A new approach to providing heat transfer from solid-state light sources is desirable.